There's plenty of engines that run exclusively on Avgas and have no issues with Lead even when it contained much more lead than it does today in 100LL.. Being synthetic these days where the Oils will mix does not mean much but infers better quality and uniformity.. Suitable additives are more often the critical factor, The Rotax requires the oil to serve a multitude of Purposes . The friction part of the Clutch(damper) the Cam of same, the reduction Gear and the General engine with Turbo Boost ( heat and load). PARTICULARLY. That is why it requires a special oil. There's a LOT of BS about what oils are best. In this case just DO what Rotax SAYS on the ones over 100HP. the 80HP and ULS can use what they always did. Sludge usually happens when engines don't get to run fully warmed up enough. Nev